Conventional mobile communication systems such as PHS (Personal Handyphone System) employ modulation methods such as a π/4-QPSK (Quadrature Phase Shift Keying) method.
FIG. 13 is a function block diagram illustrating a structure of a radio reception apparatus forming, e.g., a base station of the convention PHS employing such a modulation method.
Referring to FIG. 13, antenna 1 receives an analog receive signal of a radio frequency, and sends it to a frequency converter circuit 2, which converts it to an analog receive signal of an intermediate frequency. An analog-to-digital converter 3 converts the analog receive signal of the intermediate frequency to a digital receive signal of a predetermined sampling frequency.
The digital receive signal is applied to a quadrature detector 4, and is subjected to quadrature detection or demodulation. To be exact, the output of quadrature detector 4 is formed of an in-phase component (I-component) and a quadrature component (Q-component). For the sake of illustration, however, FIG. 13 represents the output by one signal line.
The output of quadrature detector 4 is applied to a sampling frequency converter 5, which intermittently samples the received digital receive signal (thin out the signal) to convert the sampling frequency of the digital receive signal to a lower sampling frequency.
Sampling frequency converter 5 applies its digital signal output to a band-limiting filter 6 for band limitation. Band-limiting filter 6 is a roll-off filter formed of a well known FIR (Finite Impulse Response) filter or the like.
Band-limiting filter 6 provides its output to a symbol timing detecting circuit 7 as well as a symbol timing extractor 8. Based on the digital signal output of band-limiting filter 6 and known reference data, symbol timing detecting circuit 7 detects the timing of presence of a symbol point in the applied digital signal output, and applies a timing signal indicating the detected timing to symbol timing extractor 8.
Based on the timing signal provided from symbol timing detecting circuit 7, symbol timing extractor 8 extracts data of the symbol point from the digital signal output of band-limiting filter 6.
Symbol timing extractor 8 applies the extracted data of symbol point to a carrier phase and frequency offset compensator 9, which compensates for offset in carrier phase and frequency.
Carrier phase and frequency offset compensator 9 applies its output to a determining portion 10, which determines the symbol point on an I-Q coordinate plane based on the known π/4-QPSK method, and provides a result of this determination as demodulated data.
FIG. 14 is a block diagram specifically illustrating structures of symbol timing detecting circuit 7 and symbol timing extractor 8 illustrated in FIG. 13.
Referring to FIG. 14, I- and Q-components forming the digital signal provided from band-limiting filter 6 in FIG. 13 are applied to a correlator 7a of symbol timing detecting circuit 7. Correlator 7a is also supplied with known reference data for each of I- and Q components.
Correlator 7a calculates correlation values between the band-limited I- and Q components and I- and Q-components of the reference data. An absolute value circuit 7b squares a real part of a number thus calculated, and absolute value circuit 7c squares an imaginary part thereof.
An adder 7d obtains a sum of squares of the real part and the imaginary part, and applies the sum to a maximum value detecting portion 7e as a square of the correlation value.
FIG. 15 is a graph illustrating by way of example changes in value of the square of the correlation value obtained by adder 7d. In FIG. 15, an abscissa gives the time, and the ordinate gives the square of the correlation value.
In the graph illustrated in FIG. 15, maximum value detecting portion 7e detects the timing, at which the square of the correlation value takes the maximum value, as the timing of the symbol point, and issues the timing signal, which indicates the timing of the symbol point, for each of the I- and Q-components to symbol timing extractor 8.
Symbol timing extractor 8 includes switches 8a and 8b for supplying signal components of the I- and Q-components from band-limiting filter 6 to carrier phase and frequency offset compensator 9.
These switches 8a and 8b close when maximum value detecting portion 7e issues the timing signal indicating the timing of symbol point, and thereby obtain the signal of the I- and Q-components, which are sent from band-limiting filter 6, for providing it as the signal of the symbol point. Except for the above situations, switches 8a and 8b are open.
In this manner, symbol timing detecting circuit 7 and symbol timing extractor 8 operate to extract the signal of the symbol point from the band-limited digital receive signal, and to provide it to a downstream circuit for demodulation.
The radio reception apparatus illustrated in FIG. 13 employs sampling frequency converter 5 for the following reason. Usually, the sampling frequency of the digital signal provided from analog-to-digital converter 3 depends on an intermediate frequency, which is output from frequency converter circuit 2, and must be two or more times larger than the intermediate frequency.
For effecting the band limitation on the digital signal of such as high sampling frequency (e.g., of 80 samples per symbol) by band-limiting filter 6, therefore, the FIR filter forming band-limiting filter 6 must perform an increased amount of digital signal processing, and such signal processing takes a very long time.
Therefore, sampling frequency converter 5 is provided for converting the sampling frequency (e.g., of 80 samples per symbol) of the digital signal output of quadrature modulator 4 to a lower sampling frequency (e.g., of 8 samples per symbol).
This reduces the amount of signal processing of band-limiting filter 6 as well as the time required for the signal processing.
The above manner, in which the output signal of quadrature detector 4 is simply sampled (thinning out) by sampling frequency converter 5, suffers from a problem that downstream symbol timing detecting circuit 7 and symbol timing extractor 8 cannot accurately determine the symbol point.
FIG. 15 is a graph schematically illustrating changes in value of the foregoing square of the correlation value, which is obtained by adder 7d of symbol timing detecting circuit 7 as a result of the sampling of the signal by sampling frequency converter 5 in FIG. 13. In FIG. 15, a curve of broken line represents the value of square of the correlation value, which is considered to correspond to the digital signal of a high sampling frequency provided from quadrature detector 4, and white circles on the curve represent the values of squares of the correlation values corresponding to the symbol points sampled by sampling frequency converter 5, respectively.
In the example illustrated in FIG. 15, sampling frequency converter 5 accidentally sampled the symbol point corresponding to a peak of the digital signal. Thus, the sampling points indicated by up arrows in FIG. 15 were correctly sampled by accident.
In general, however, sampling frequency converter 5 does not particularly recognize the timing of the sampling to be performed, and performs the sampling in accordance with arbitrary timing.
FIG. 16 is a graph schematically illustrates another example of changes in value of the foregoing square of the correlation value, which is obtained by adder 7d of symbol timing detecting circuit 7 as a result of the sampling of the signal by sampling frequency converter 5 in FIG. 13. Similarly to FIG. 15, a curve of broken line in FIG. 16 represents the value of square of the correlation value, which is considered to correspond to the digital signal of a high sampling frequency provided from quadrature detector 4, and marks of “X” on the curve represent the values of squares of the correlation values corresponding to the symbol points sampled by sampling frequency converter 5. In the example illustrated in FIG. 15, sampling frequency converter 5 accidentally and correctly sampled the symbol point corresponding to the peak of the digital signal. In the example illustrated in FIG. 16, however, sampling frequency converter 5 did not correctly sample the symbol point corresponding to the peak. Thus, the sampling points indicated by up arrows in FIG. 16 are deviated from the correct sampling timing in FIG. 15.
Since the sampling timing of sampling frequency converter 5 is not particularly controlled, the sampling may be performed in accordance with deviated timing as illustrated in FIG. 16, and the results thus obtained are applied to the downstream symbol timing detecting circuit.
However, a conventional mobile communication system such as PHS generally employs the well known π/4-QPSK method as a modulation method. Therefore, even if the timing of sampled symbol points is deviated from the correct timing, a reception error or the like does not occur during signal processing.
This will now be described more specifically. According to the π/4-QPSK modulation method, as is well known, only one symbol point is arranged in each of quadrants on an I-Q coordinate plane. In this method, since only one symbol point is present in each quadrant, a symbol point is recognized as a correct point as long as the symbol point is in the correct quadrant even if there is a slight deviation in timing of the symbol point. Therefore, the reception error does not occur in subsequent signal processing.
Recent mobile communication systems such as data communication require higher quality and capacity of transmission than those in conventional voice communication. Therefore, it has been studied to apply modulation systems of more values than π/4-QPSK method already described.
As an example of such multivalued modulation method, a known 16-QAM (Quadrature Amplitude Modulation) method has been practically used in a certain kind of data communication. According to this 16-QAM method, as is well known, the I-Q coordinate plane includes four signal points, which are arranged in a grid-like fashion, in each quadrant, and thus totally includes 16 signal points, and a symbol point of the receive signal corresponds to any one of the 16 signal points.
In the case where the 16-QAM method is employed as the modulation method of PHS, therefore, if the symbol points are sampled in accordance with improper timing as illustrated in FIG. 16, it is impossible to correctly recognize the correct symbol point, which corresponds to one of the four symbol points in a grid-like fashion in a quadrant of the I-Q coordinate plane, and a symbol point neighboring to the correct symbol point is erroneously recognized instead of the correct symbol point. Thereby, a reception error occurs in subsequent signal processing.
Accordingly, an object of the invention is provide a radio reception apparatus, a symbol timing control method and a symbol timing control program, which can prevent a reception error due to a deviation of sampling timing of a symbol point even in such a case that a mobile communication system employs a multivalued modulation method for enabling fast and large-capacity transmission.